In photolithography used for manufacture of a semiconductor, a photoresist composition based on various novolac-based polymers and acrylic-based polymers, oxystyrene-based polymers including hydroxystyrene, or the like is used to form, for example, a thin film of a composition for photo lithography such as photoresists and anti-reflection films on a substrate, such as a silicon wafer, or the like, then excimer laser light, etc. is irradiated via a mask with a drawn circuit pattern of a semiconductor device, and by etching the substrate using the photoresist pattern obtained by image development as a protective layer, a fine pattern is formed, which corresponds to the semiconductor circuit, on the surface of the substrate. Along with the increasing accumulation degree, formation of a finer pattern is called for, and currently, lithography techniques by KrF excimer laser light (wavelength of 248 nm) and ArF excimer laser light (wavelength of 193 nm) are used in commercial production. Research and development are in progress also for lithography techniques by F2 excimer laser light with a shorter wavelength (wavelength of 157 nm), EUV (extreme ultraviolet) of a shorter wavelength than those excimer lasers, X-rays, and electron beams.
The above-described various polymers are also used as a functional high-molecular material in various industrial fields, and particularly in the field of electronic materials, they are used as a raw material of a photosensitive resin component, especially a resin component for a semiconductor resist used for an interlayer insulating film such as a semiconductor element, and further as a material for a flat panel display. In recent years, also in fields such as a flat panel display using crystalline liquid and organic EL, enhancement of performance and review for improvement are in progress of optical and electronic parts such as definition enhancement, view angle enhancement, image quality enhancement, high luminance of a light source by use of an optical semiconductor such as light-emitting diode (LED), shortening of wavelengths, whitening, and moreover, frequency enhancement of electronic circuits, and circuits and communications using light. Moreover, progress in the technical field of semiconductors is remarkable, and electronic devices are in rapid progress for downsizing and weight saving, performance enhancement, and multi-functionalization. In order to correspond to such progress, wiring boards need to be highly densified and highly wired. The semiconductor-related material such as various photoresists, lower layer films, interlayer insulating films, or the like, and a material for displays used for an electronic substrate, a semiconductor circuit, a display, or the like, which are designed to be in high density by the fine process as described above are required to have metal ions contained in those high-molecular material suppressed to an extremely small amount. Thus, there is a need for reducing the metal content of polymers, intermediates, and monomers thereof.
Furthermore, while miniaturization of a semiconductor circuit associated with an increasing accumulation degree is in progress, requests are more demanding to reduce the amount of impurities contained in a copolymer used for semiconductor lithography. Especially, metal impurities must be removed as much as possible since they have various adverse influences on manufacture of a semiconductor. For example, when metal impurities, such as sodium and iron, are contained in a copolymer for a chemistry amplification type resist, the metallic component will capture the acid substance generated from an acid generating agent at the time of exposure, and the copolymer which is a substrate component of the resist does not fully dissolve, resulting in failure to form a desired pattern. Moreover, when the metal impurities contained not only in the copolymer for a resist but also in a copolymer for semiconductor lithography, such as a copolymer for a topcoat and a copolymer for an anti-reflection film, ultimately remains at the surface of the semiconductor substrate, the electrical property of the semiconductor will be impaired, resulting in reduction of product yield.
As a method for removing metal impurities in a copolymer for an electronic material, for example, reported are a method wherein the copolymer is extracted using an organic solvent and water and distributed to an organic layer; metal is distributed to an aqueous layer; and the aqueous layer is removed (patent document 1) and a method wherein in an organic solvent solution of a cycloaliphatic hydrocarbon polymer, a poor solvent of such polymer and acid are mixed to solidify the polymer, followed by mixing of the solidified polymer, an insoluble organic solvent, acid, and water to extract metal (patent document 2). Moreover, reported are a method wherein a novolac resin solution passes through a cationic exchange resin and an anion exchange resin which were washed with deionized water and a mineral acid solution (patent document 3), a method wherein a dispersion liquid in which the polymer is dispersed to a dispersion medium is filtered with filters, such as a filter cloth washed beforehand with an acidic aqueous solution, to obtain wet powder of the polymer having a reduced content of metal (patent document 4), and a method wherein a polymer solution passes through an adsorbent, such as a compound between layers of clay, activated carbon, and silica gel, or the like, to remove metal (patent document 5). Other method is reported, wherein a soluble compound capable of forming a complex in an equivalent amount or more of the metal impurities in a polymer is added to a solution of a polymer for a resist and washed with pure water after the reaction had completed (patent document 6). However, these methods are complicated in operation and were difficult to apply for manufacturing copolymers in a commercial scale.